I have various rules of thumb for when it’s too hot to do certain things. Over 80? Too hot to hike (sweat + overnight cobweb construction + lots of body hair makes me feel like a wad of cotton candy after a mile or so). I draw the line slightly higher, at 85, for running, mainly because of the skimpy apparel involved. I’ll paddle into the low 90s, but not on open, unshaded water. I can handle 90 degrees on a mountain bike; the calculation becomes more involved on a road bike. I’ll ride up to 95 on road, maybe higher if I don’t have to stop; few things are more demoralizing than coming to a stoplight after generating an 18-mile-per-hour breeze, then losing that breeze altogether. (Sweaty fact: 109 F is 42.777 C.)